In these years, optical coherence tomography (OCT) systems (referred to as OCT systems, below) that utilize interference due to low coherence light have been practically used. The OCT systems can noninvasively capture a tomograph image of a test sample with high resolution. The OCT systems have thus been growing into indispensable systems, particularly in ophthalmology, to obtain a tomograph image of the ocular fundus of the target eye. Besides ophthalmology, the OCT systems are used for purposes such as for observing a tomograph of the skin or capturing a tomograph image of the wall surfaces of the digestive organ or the circulatory organ in the form of an endoscope or a catheter.
An ophthalmologic OCT system that can capture functional OCT images besides normal OCT images (also referred to as intensity images) has been developed where the normal OCT images image the shape of ocular fundus tissues while the functional OCT images image optical characteristics, actions, or other information of ocular fundus tissues. Particularly, polarization-sensitive OCT systems that can draw a nerve fiber layer or a retinal layer have been developed as one of functional OCT systems and the systems for diseases such as glaucoma or age-related macular degeneration have been increasingly studied.
A polarization-sensitive OCT system can form a polarization OCT image using a polarization parameter (retardation and orientation), which is one of optical characteristics of ocular fundus tissues, to discriminate or segment the ocular fundus tissues. Generally, a polarization-sensitive OCT system includes an optical system in which a wave plate (for example, λ/4 wave plate or λ/2 wave plate) is used to appropriately change the polarization states of light measured by the OCT system and reference light. A polarization-sensitive OCT system forms a polarization OCT image by controlling polarization of light emitted from a light source, using light that has been modulated into a desired polarization state as a measurement light beam for observing a specimen, splitting an interference light beam into two orthogonal straight polarized light beams, and detecting the polarized light beams (NPL 1: Biomedical Optics Express 3 (11), Stefan Zotter et al. “Large-field high-speed polarization sensitive spectral domain OCT and its applications in ophthalmology”).
As a method for controlling polarization, a method for modulating a polarized state using an electro-optic modulator (EOM) has been provided (NPL 2: Optics Express 5 (15), Barry Cense et al. “Polarization-sensitive spectral-domain optical coherence tomography using a single line scan camera”). This method enables formation of polarization OCT images on the basis of polarization information of multiple polarized states by applying light beams of multiple polarized states to the same position, whereby more accurate polarization OCT images can be captured.
Meanwhile, size reduction of OCT systems is required at medical institutions with the needs of installing various inspection devices. Thus, a polarization-sensitive OCT system that is smaller than and has a more flexible optical system than existing systems by including an optical fiber as an optical system has been developed (NPL 3: Journal of Biomedical Optics 19 (2), Hermann Lin et al. “All fiber optics circular-state swept source polarization-sensitive optical coherence tomography”).
Existing polarization-sensitive OCT systems include components such as a polarization maintaining (PM) fiber (referred to as a PM fiber, below), a wave plate, and an EOM for controlling polarization. Components such as PM fibers, wave plates, and EOMs, however, are extremely expensive and consequently make the polarization-sensitive OCT expensive. Moreover, conventional OCT systems are unable to easily accept additional components and an additional polarization-OCT-image capturing function. Thus, medical institutions that have already had a conventional OCT system have to purchase a new system, causing a heavy burden. Furthermore, a large space is required to install two OCT systems.
NPL 1 discloses the configuration of a polarization-sensitive OCT system that includes an interferometer for which a PM fiber is used and a wave plate for controlling polarization of a measurement light beam and a reference light beam. This OCT system can facilitate polarization adjustment but cannot be formed at a low cost due to the use of expensive optical elements.
NPL 2 discloses a polarization-sensitive OCT system including an EOM for controlling polarization. However, as in the system of NPL 1, the polarization-sensitive OCT system cannot be formed at a low cost because of a very expensive EOM.
NPL 3 discloses a polarization-sensitive OCT system including an interferometer for which a PM fiber is used. The use of the PMF reduces the size of the system but prevents cost reduction of the system because the PMF is a very expensive optical component.
In addition, the polarization-sensitive OCT systems of the above-described NPLs have configurations basically different from the configuration of conventional OCT systems and thus conventional OCT systems are highly unlikely to be extensible, specifically, to be changed into polarization-sensitive OCT systems. Changing conventional OCT systems into the above-described polarization-sensitive OCT systems involves replacement or addition of most of the components, making it impossible to easily add functions. Consequently, a polarization-sensitive OCT system is installed along with a conventional OCT system, preventing space saving.